Field of the Invention
The invention relates to a feed pump with a driven impeller which rotates in a pump casing and which has, on at least one of its end faces, a ring of guide vanes delimiting vane chambers, and with at least one part-annular channel which is arranged in the pump casing in the region of the guide vanes and which forms, with the vane chambers, a feed chamber provided for feeding a liquid from an inlet port to an outlet port and has a compression region with a cross section tapering over a limited angular sector.
Such feed pumps are known as peripheral or side-channel pumps and are often used for feeding fuel from a fuel tank to an internal combustion engine of a motor vehicle. In this case, the guide vanes generate in the feed chamber a circulating flow which runs transversely to the direction of movement of the guide vanes. The compression region serves for increasing the pressure in the feed chamber. As a result, gas bubbles of vaporous fuel which are present in the hot fuel condense in the liquid fuel due to the vapour pressure being exceeded. This is important, for example during hot-starting of the motor vehicle, since, in this case, the temperature of the fuel is particularly high and gas bubbles are therefore very often sucked in through the inlet port. Cold fuel, which usually does not contain any gas bubbles, is likewise to be conveyed reliably by means of the feed pump.
In a feed pump known from practice, the compression region is located directly at that region of the part-annular channel which adjoins the inlet port. The fuel thereby flows from the inlet port directly into the compression region and generates turbulence there. However, this turbulence has regions with high and low pressures and consequently prevents the gas bubbles from being dissolved reliably. Furthermore, in this feed pump, there is the risk of cavitation in the compression region, particularly on the wall of the part-annular channel. This cavitation eventually leads to destruction of the wall of the part-annular channel and to a reduced delivery of the feed pump.
A further fuel pump is known, which has a region of constantly changing cross section between the inlet port and the compression channel (DE 196 07 573 A1). The gas contained in the fuel is led radially inwards by displacement, collected in pockets and discharged via a degassing bore.
Another known fuel pump possesses an elongate vapour channel, the cross section of which is reduced in the last section (U.S. Pat. No. 5,284,417). In this case, the reduction in cross section is designed in such a way that it leads to a degassing bore. The fuel/gas mixture carried in the elongate vapour channel is thus led to the degassing bore, so that the gas is discharged via this bore.
Another feed pump has become known, in which that region of the part-annular channel which adjoins the inlet port has initially a first region with a constant cross section. At the end of this region, a degassing bore is worked into the pump casing at the radially inner end of the part-annular channel. The cross section of the part-annular channel subsequently decreases abruptly, the said channel then being led with a constant cross section as far as the region of the outlet port (U.S. Pat. No. 4,591,311). In this case, the gas bubbles are not condensed in the fuel, but are to be discharged through the degassing bore. Since the circulating flow is already forming in the first region, the gas bubbles are entrained by the fuel and form a foam with the liquid fuel. This rules out a reliable separation of gas bubbles and fuel and, consequently, discharge of the gas bubbles through the degassing bore. Furthermore, the fraction of gas bubbles in the fuel to be conveyed is exposed to pronounced time fluctuations, the result of this, particularly in the region of the abrupt changing cross section of the part-annular channel, being cavitation on the pump casing and, consequently, a decrease in the delivery of the feed pump.
The problem on which the invention is based is to design a feed pump of the type mentioned in the introduction, in such a way that gas bubbles present in the fuel are condensed particularly reliably and that, if possible, no cavitation occurs in the region of the part-annular channel.